disk check your HDD for errors. sounds like it's slowly dying. if that's not the case, try to run memtest and see if your ram's still ok.
if your only problem now is the sound, then try updating the driver, if you haven't. if it still persists; then best case scenario your audio is dying and that would require a new motherboard i guess. if you still feel the PC's still lags at more simpler tasks, then, it may be the motherboard